A. Field of the Invention
The present invention relates to the field of animal health and in particular to Lawsonia intracellularis. In particular, the invention relates to a method of diagnosing Lawsonia intracellularis infection and a diagnostic test kit using Lawsonia intracellularis-specific antibodies. The invention also relates to the use of the method or test kit for diagnosing Lawsonia intracellularis infections.
B. Background of the Invention
L. intracellularis, the causative agent of porcine proliferative enteropathy (“PPE”), affects virtually all animals, including humans, rabbits, ferrets, hamsters, fox, horses, and other animals as diverse as ostriches and emus. L. intracellularis is a particularly great cause of losses in swine herds in Europe as well as in the United States.
A consistent feature of PPE is the occurrence of intracytoplasmic, non-membrane bound curved bacilli within enterocytes in affected portions of intestine. The bacteria associated with PPE have been referred to as “Campylobacter-like organisms.” S. McOrist et al., Vet. Pathol., Vol. 26, 260-264 (1989). Subsequently, the causative bacteria have been identified as a novel taxonomic genus and species, vernacularly referred to as Ileal symbiont (IS) intracellularis. C. Gebhart et al., Int'l. J. of Systemic Bacteriology, Vol. 43, No. 3, 533-538 (1993). More recently, these novel bacteria have been given the taxonomic name Lawsonia (L.) intracellularis. S. McOrist et al., Int'l. J. of Systemic Bacteriology, Vol. 45, No. 4, 820-825 (1995). These three names have been used interchangeably to refer to the same organism as further identified and described herein.
L. intracellularis is an obligate, intracellular bacterium which cannot be cultured by normal bacteriological methods on conventional cell-free media and has been thought to require attached epithelial cells for growth. S. McOrist et al., Infection and Immunity, Vol. 61, No. 19, 4286-4292 (1993) and G. Lawson et al., J. of Clinical Microbiology, Vol. 31, No. 5, 1136-1142 (1993) discuss cultivation of L. intracellularis using IEC-18 rat intestinal epithelial cell monolayers in conventional tissue culture flasks. In addition, H. Stills, Infection and Immunity, Vol. 59, No. 9, 3227-3236 (1991) discusses using Intestine 407 human embryonic intestinal cell monolayers and GPC-16 guinea pig colonic adenocarcinoma cell monolayers in conventional tissue culture flasks.
In particular, L. intracellularis can be cultivated be methods known in the art, preferably, according to U.S. Pat. Nos. 5,714,375 and 5,885,823. For example, culture cells may first be inoculated with an inoculum comprising L. intracellularis bacteria so as to infect the cells with the bacteria. Numerous cell lines can be used in practicing the invention, including, but not limited to, IEC-18 (ATCC 1589)—rat intestinal epithelial cells, HEp-2 (ATCC 23)—human epidermoid carcinoma cells, McCoys (ATCC 1696)—mouse (non-specified) cells, BGMK (Biowhittaker #71-176)—buffalo green monkey kidney cells, and swine intestinal epithelium cells. The preferred culture cells are HEp-2, McCoys or IEC-18 cells.
If culture cells are used, prior to being inoculated, the cells may be in the form of a monolayer. To form a monolayer, the cells may be seed into conventional flasks. Each flask is generally seeded with between about 1×105 cells to about 10×105 cells per 25, 75, 150, 850 cm2 flask or roller bottle mixed with growth media. The growth media may be any media for cell cultivation which includes a nitrogen source, necessary growth factors for the chosen culture cells, and a carbon source, such as glucose or lactose. The preferred media is DMEM fortified with Ham's F 12 with 1-5% fetal bovine serum, although other commercially available media may be used with good results.
Successful cultivation of L. intracellularis is enhanced by maintaining the culture cells in a constant state of growth. Therefore, the culture cell monolayer should be at about 20 percent to about 50 percent confluency at the time of inoculation. Preferably, the cells should be at about 30 percent to about 40 percent confluency at the time of inoculation, most preferably at about 30 percent confluency.
Alternatively, the cells, prior to being inoculated, may be grown in suspension, as described infra. Preferably, the cells are first grown to 100% confluency in the form of a monolayer in an adherent type system, e.g., a roller bottle system, and then transferred to 3-3000 liters and grown in suspension.
The inoculum may be a culture of L. intracellularis obtained from infected swine or other animals.
The inoculum can be an intestinal homogenate prepared by scraping the mucosa off of the ileum of a swine or other animal infected with PPE. When preparing an intestinal homogenate, ileal sections selected for culture should show severe lesions with gross thickening of the gut. Due to the fragile nature of the bacteria, samples should preferably be stored at −70° C. as quickly as possible after necropsy. An antibiotic to which L. intracellularis is resistant such as Vancomycin, Amphotericin B or members of the aminoglycoside group of antibiotics, including Gentamicin and Neomycin, to name a few, is preferably added to the inoculum to suppress contaminating bacteria while permitting L. intracellularis growth. Whether the inoculum is a pure culture or an intestinal homogenate, inoculation of the culture cells can be performed by various techniques known in the art, given the teachings herein.
The bacteria and/or inoculated culture cells are then incubated under a reduced dissolved O2 concentration. At dissolved oxygen concentrations greater than 10% L. intracellularis growth is less than optimal with cessation of growth eventually occurring at oxygen concentrations outside this range. Preferably, the bacteria and/or inoculated culture cells are incubated in a dissolved oxygen concentration in the range of about 0% to about 10%. More preferably, the bacteria and/or cells are incubated in an oxygen concentration in the range of about 0% to about 8%, with an oxygen concentration of about 0% to about 3.0% being most preferred.
The proper concentration of carbon dioxide is also important to the proper growth of L. intracellularis. At carbon dioxide concentrations greater than 0% and less than 4%, non-optimum growth occurs with cessation of growth eventually occurring at carbon dioxide concentrations outside this range. Preferably, the carbon dioxide concentration is in the range from about 6% to about 10%, with a carbon dioxide concentration of about 8.8% being most preferred.
In addition, the cells are preferably incubated at a hydrogen concentration in the range from about 73% to about 96%. Nitrogen may be used in place of some or all of the hydrogen present. Most preferably, the cells are incubated in about 0 to about 8.0% O2, about 8.8% CO2, and about 83.2% H2.
Inoculated cells may be incubated in a dual gas incubator or other gas chambers which contains the proper hydrogen, oxygen and carbon dioxide concentrations and which allows the cells to be suspended during incubation. The chamber should comprise a means for maintaining the inoculated cells in suspension, and a gas monitor and supply source to supply and maintain the proper gas concentrations. The incubation temperature should be in the range of from 30° C. to about 45° C. and is more preferably in the range of from about 36° C. to about 38° C. Most preferably, the temperature is about 37° C. The necessary equipment for cultivation and attenuation is readily available to those or ordinary skill in the art given the teachings herein. One example of equipment suitable for carrying out the resent invention is a dual gas incubator, e.g., model 480 (Lab-Line, Melrose Park, Ill.) in conjunction with spinner flasks to maintain the cells in suspension. The presently preferred equipment comprises a fermentor, bioreactor, stir plate or rotary shaker containing media and capable of maintaining the culture cells in suspension via sparging gas of the appropriate concentration, or other means of mechanical agitation, and continuously monitoring dissolved O2 levels in the media. New Brunswick, Braun and other companies make suitable fermentors and bioreactors for this purpose.
By maintaining the inoculated cells in a suspended state during incubation, maximum growth of the cells, and hence L. intracellularis, is achieved by increasing each individual cell's exposure to growth media and the proper mixture of hydrogen, oxygen and carbon dioxide. The culture cells can be agitated and maintained in suspension by a variety of methods known in the art including, for example, culture flasks, roller bottles, membrane cultures, biobags, WAVE™ bioreactor systems, and spinner flasks. The cells may be kept in suspension during incubation by incubating the cells in a spinner flask inside a dual gas incubator or similar apparatus. The term “spinner flask”, as used herein, means a flask or other container which employs a paddles, propeller or other means to agitate the culture and keep the cells contained therein in suspension.
Alternatively, the inoculated cells are incubated until the cells reach confluency and then the cells are placed in a spinner flask containing growth media and incubated in a dual gas incubator while spinning the flask. Preferably, the inoculated cells are scraped or trypsinized and passaged into the spinner flask. This can be achieved by a variety of methods known in the art such as using a cell scraper to detach the cells. Once the cells are introduced into the spinner flask, the paddle of the spinner flask is typically rotated in the range of from about 30 to about 60 rpm on a magnetic stir plate in order to maintain the infected cells in suspension.
A portion of the cultivated L. intracellularis is then passaged to fresh culture to increase the production of L. intracellularis bacteria. The term “passaging” or variations thereof herein means the process of transferring a portion of the cultivated L. intracellularis to fresh culture cells in order to infect the fresh cells with the bacterium. The term “fresh”, as used herein, means cells which have not yet been infected by L. intracellularis. Preferably such cells are on the average no more than approximately one day old.
The passage of L. intracellularis in suspension cultures may be accomplished by removing a portion of the original culture and adding it to a new flask containing fresh culture cells. If the original culture has a high number of bacteria/ml, for example, greater than about 104 bacteria/ml, it is preferable to add between about 1 to 10% (volume to volume) of culture from the infected flask to a new flask containing fresh cells. This is preferably done when 50-100% of the cells are infected. If fewer than 50% of the cells are infected, passaging is preferably accomplished by splitting the culture 1:2 into a new flask and scaling-up the volume by adding fresh media. In either case, cell lysis and other steps are not required, in direct contrast to the passage of monolayer cultures, as in the prior art.
After sufficient growth of the culture cells and subsequent infection by L. intracellularis, as determined by indirect fluorescent antibody (IFA) staining, TCID50 or another comparable method, at least a portion of the cultivated L. intracellularis bacteria is then harvested. The harvesting step may be performed by separating the bacteria from the suspension by various techniques known to those of ordinary skill in the art, given the teachings herein. Preferably, the L. intracellularis bacteria is harvested by centrifuging the contents of all or a portion of the suspension to pellet the culture cells, resuspending the resulting cell pellets, and lysing the infected cells. Typically, at least a portion of the contents is centrifuged at about 3000×g for about 20 minutes in order to pellet the cells and bacteria. The pellet may then be resuspended in, for example, a sucrose-phosphate-glutamate (SPG) solution and passed approximately 20 times through a 25 gauge needle in order to lyse the cells. If further purification is desired, the samples can be centrifuged at about 145×g for about five minutes to remove cellular nuclei and debris. The supernatant may then be centrifuged at about 3000×g for about twenty minutes and the resulting pellet resuspended in an appropriate diluent, such as SPG with fetal bovine serum (to prepare harvested bacteria suitable for lyophilization, freezing, or use as an inoculant) or growth media (to prepare harvested bacteria more suitable for passaging to fresh cells).
As previously mentioned, effective growth of L. intracellularis for large-scale production is enhanced by keeping the tissue cells actively growing. Using suspension cultures greatly facilitates keeping the cells actively growing and permits continuous culture expansion and scale-up. Using a fermentor and between about 0 to 3% dissolved O2 as explained above, enables growth of up to 108 bacteria/ml.
When using McCoys or IEC-18 cells, it is preferable to add gelatin, agarose, collagen, acrylamide or silica beads, such as Cultisphere-G porous microcarriers (HyClone Laboratories, Logan Utah), along with the growth media. However, HEp-2 cells and others do not require microcarriers according to the methods used herein.
For culture maintenance purposes, with HEp-2 cultures, preferably 25% to 50% of the culture is removed and replaced with fresh media at weekly intervals. For cell cultures with microcarriers or beads, preferably 25% to 50% of the culture is removed and replaced with fresh media 1-2 times weekly. For scale-up purposes, an additional 25% to 50% of media, or media with microcarriers, may be added to the culture.
Depending upon the rate at which the culture cells become infected, passage to fresh cells generally occurs between about every 2 to about 7 days. Assuming that the culture cells become at least 70% infected within 2 to 7 days, preferably passage occurs between about every 5 to 7 days.
Diagnosis of L. intracellularis antigen is carried out today by using direct immuno-fluorescence and PCR. Diagnosis of antibodies specific to L. intracellularis is carried out today by using immuno-fluorescence. These methods are laborious and time consuming and are not suitable for large scale screenings.
Effective diagnosis of PPE has also been hindered by the time required to culture the causative bacteria. As a result of the present invention, development of diagnostic tools promoting rapid and accurate assays for the presence of L. intracellularis in biological samples taken from swine and other animals susceptible to PPE is now possible.
Therefore, the technical problem underlying the present invention is to provide improved methods for diagnosis of L. intracellularis disease.